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pe mercial Tem Extended Com 0C to 70C -2 quipment Handheld E for Portable
FEATURING ture Range ra
JULY 2000
ML4876*
LCD Backlight Lamp Driver with Contrast
GENERAL DESCRIPTION
The ML4876 is an ideal solution for driving small cold cathode fluorescent tubes (CCFL) used in liquid crystal display (LCD) backlight applications. It provides the dimming ballast control and the contrast control for the LCD display. By utilizing differential drive the ML4876 can deliver the same light output with significantly less input power compared to existing single ended drive schemes. Improvements as high as 30% can be realized when using low power lamps and advanced LCD screen housings. This increased light output is achieved because the differential drive configuration is much less sensitive, and therefore less power is wasted in the capacitive parasitics that exist in the backlight housing. An additional benefit of this configuration is an even distribution of light. The ML4876 is optimized for portable applications where high efficiency is critical to maximize battery life. The high efficiency is achieved by a resonant scheme with zero voltage switching. * THIS PART IS END OF LIFE AS OF JULY 1, 2000
FEATURES
s s s s s s s s
Backlight lamp driver with differential drive Up to 30% lower power for same light output Low standby current (< 10A) Improved efficiency (95%) Allows all N-channel MOSFET drive Low switching losses Resonant threshold detection Buck regulator uses synchronous rectification
BLOCK DIAGRAM
13 VDD HVDD 15 6 ON/OFF VREF 5 MASTER BIAS & UVLO LINEAR REGULATOR ONE SHOT VDD DR3 DR1 DR1 DR2 NEG EDGE DELAY SQ RQ LON 4 + QS - QR 16V - + F GATE 12 F ILIM 20 0.1V FEA- 2 FEA+ 3 + - - - + + FEA OUT 1 9 CT 8 LEA OUT CLK OSCILLATOR QS QR RESONANT THRESHOLD DETECTOR 0.5V 17 GND 18 B ON 19 B OFF 11 B SYNC OUT L RTD 10 L GATE 1 14
Q T Q DR2
L GATE 2 16
- - + + 0.2V LEA-
7
1
ML4876
PIN CONFIGURATION
ML4876 20-Pin SSOP (R20)
FEA OUT FEA- FEA+ L ON VREF ON/OFF LEA- LEA OUT CT L RTD 1 2 3 4 5 6 7 8 9 10 20 19 18 17 16 15 14 13 12 11 F ILIM B OFF B ON GND L GATE2 HVDD L GATE1 VDD F GATE B SYNC OUT
TOP VIEW
PIN DESCRIPTION
PIN NAME DESCRIPTION PIN NAME DESCRIPTION
1 2 3 4
FEA OUT FEA- FEA+ L ON
Output of flyback (contrast) error amplifier Negative input of flyback (contrast) error amplifier Positive input of flyback (contrast) error amplifier Logic input. A "0" on this pin disables the lamp driver section only Voltage reference output
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B SYNC OUT
Output of MOSFET driver. Connects to gate of synchronous FET catch diode. Connects to gate of MOSFET in primary side of contrast control Output of linear regulator. Positive power for IC. Output of MOSFET driver. Connection to gate of one side of inverter FET drive pair Battery power input to linear regulator Output of MOSFET driver. Connection to gate of one side of inverter FET drive pair Ground Connection to primary side of gate pulse transformer Output of MOSFET driver. Connection to gate of FET that disables the input power. Input to current limit comparator
12 13 14
F GATE VDD L GATE1
5 6 7 8 9 10
VREF ON/OFF LEA- LEAOUT CT L RTD
15 Logic input. A "0" on this pin disables the linear regulator 16 Negative input for lamp error amplifier Output of lamp error amplifier Oscillator timing capacitor Input to resonant threshold detector 19 17 18
HVDD L GATE2
GND B ON B OFF
20
F ILIM
2
ML4876
ABSOLUTE MAXIMUM RATINGS
Absolute maximum ratings are those values beyond which the device could be permanently damaged. Absolute maximum ratings are stress ratings only and functional device operation is not implied. Supply Current (ICC) ............................................... 75mA Output Current, Source or Sink ............................. 250mA Voltage on HVDD ..................................................... 20V Current into L RTD ............................................... 10mA Transient Voltage on B ON .......................................... 9V Voltage on Any Other Pin ............... -0.3V to VDD +0.3V Junction Temperature .............................................. 150C Storage Temperature Range ..................... -65C to 150C Lead Temperature (Soldering 10 sec.) ..................... 260C Thermal Resistance (qJA) Plastic SSOP ............... 100C/W
OPERATING CONDITIONS
Temperature Range ML4876C ................................................... 0C to 70C ML4876E ............................................... -20C to 70C
ELECTRICAL CHARACTERISTICS
Unless otherwise specified, VDD = 5V 5%, CT = 47pF, TA = Operating Temperature Range (Note 1)
SYMBOL CURRENT REGULATOR Error Amplifier
Open Loop Gain Bias Point Output High Output Low Bandwidth (-3dB) Input Voltage Range Input Bias Current -0.3 Closed loop ILOAD = 5A ILOAD = 25A 60 0.18 2.8 70 0.2 3.0 0.4 1 0.2 50 VREF 100 0.7 0.22 dB V V V MHz V nA
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Current Limit Comparator
Current Threshold Input Bias Current Propagation Delay VLILIM = 0.1V (Note 2) 450 500 50 30 550 100 mV nA ns
Output Drivers
Output High - B SYNC OUT, B OFF Output Low - B SYNC OUT, B OFF Rise & Fall time - B SYNC OUT, B OFF Output High - B ON Output Low - B ON Fall Time - B ON ONE SHOT Pulse Width DELAY TIMER Delay Time VDD = 5V, ILOAD = 12mA ILOAD = 12mA CLOAD = 100pF VDD = 5V, ILOAD = 12mA ILOAD = 50mA CLOAD = 2400pF (Note 2) 100 20 4.625 4.625 4.8 0.2 20 4.8 0.2 45 150 35 0.375 80 200 55 0.375 50 V V ns V V ns ns ns
FLYBACK REGULATOR Error Amplifier
Open Loop Gain Offset Voltage Output High Output Low ILOAD = 5A ILOAD = 25A 60 -15 2.8 3.0 0.4 0.7 70 15 dB mV V V
3
ML4876
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER FLYBACK REGULATOR (Continued) Current Limit Comparator
Threshold Input Bias Current Propagation Delay VLILIM = 0.1V 70 100 50 125 130 100 250 mV nA ns
(Continued)
CONDITIONS MIN TYP MAX UNITS
Output Drivers
Output High - F Gate Output Low - F Gate Rise & Fall Time VDD = 5V, ILOAD = 12mA ILOAD = 50mA CLOAD = 1000pF 4.625 4.8 0.2 20 0.375 50 V V ns
HIGH VOLTAGE INVERTER Oscillator
Nominal Frequency Discharge Current Peak Voltage Valley Voltage Output Drivers Output High - L GATE 1, 2 Output Low - L GATE 1, 2 Rise & Fall Time - L GATE 1, 2 VDD = 5V, ILOAD = 12mA ILOAD = 50mA CLOAD = 1000pF 4.625 4.8 0.2 20 0.375 50 V V ns VCT = 2V 59 500 2.3 0.8 70 700 2.5 1 81 900 2.7 1.2 kHz A V V
Resonant threshold Detector
Threshold Hysteresis 0.75 250 1.1 500 1.45 750 V mV
Lamp Out Detect
Threshold 16 18 20 V
Under Voltage Detector
Start Up Threshold Hysteresis 3.8 150 4.1 300 4.4 450 V mV
Logic Interface (On/Off, L ON)
VIH VIL Input Bias Current VI = 3V 10 2.6 0.5 25 V V A
Linear Regulator
Regulator Voltage (VDD) Regulator Source Current Drop Out Voltage Drop Out Voltage HVDD Input Voltage Range HVDD = 12V External to device IHVDD = 1mA IHVDD = 5mA 5 4.75 5.0 10 30 125 90 275 18 5.35 V mA mV mA V
4
ML4876
ELECTRICAL CHARACTERISTICS
SYMBOL SUPPLY
VDD Supply Current (No Load) VDD Supply Current VDD Supply Current VREF Output Voltage VREF Load Regulation VREF Line Regulation VREF Line, Load, Temp
Note 1: Limits are guaranteed by 100% testing, sampling, or correlation with worst case test conditions. Note 2: Actual load is 1200pF. The 2:1 transformer reflects an effective 2400pF.
(Continued)
CONDITIONS MIN TYP MAX UNITS
PARAMETER
On/Off = 3V, LON = 3V On/Off = 3V, LON = 0V On/Off = "0", HVDD = 12V TA = 25C IVREF = 25A 2.47
0.45 200
0.6 350 10
mA A A V mV mV V
2.5 10 20
2.53 20 30 2.535
2.465
2.5
5
ML4876
INPUT 5V to 18V T3 CONTRAST VOLTAGE -20V MAX. (ADJUSTABLE) D1 C2 + 22F 25V C3 1.0F R1 750k Q1 C1 + 22F 25V R2 1.6M Q2 NOTE 1 5V REF C4 10F 16V D2 1N4148 C12, 0.1F C10 39pF 1kV T1 U2-A L1
R12
D3 10V
+
T2 C9 0.1F 1N5817
R3 100k
Q3 C8, 0.1F
Q4
Q5
C11 0.1F 20 19 18 17 16 15 14 13 12 11
R16
ML4876
1 R4 200k C5 C6 0.47F 47F R5 200k C7 47pF 2 3 4 5 6 7 8 9 10
R8 4.3k
R6 200k R7 1.6M
CONTRAST ADJUST
LON
ON/OFF
BRIGHTNESS ADJUST
NOTE 1 R1, D3, Q2 ARE OPTIONAL AND ALLOWS A BATTERY VOLTAGE RANGE FROM 7V TO 28V. REMOVING THESE COMPONENTS AND CONNECTING DIRECTLY TO THE INPUT VOLTAGE ALLOWS 5.0 TO 18V.
Figure 1. ML4876 Typical Application Schematic
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LAMP
U2-B
ML4876
FUNCTIONAL DESCRIPTION
The ML4876 consists of a PWM regulator, a lamp driver/ inverter, a linear regulator, a flyback regulator, and control circuits. This IC, in conjunction with external components, converts a DC battery voltage into the high voltage and high frequency ac signal required to start and drive miniature cold cathode fluorescent lamps. In addition it generates the DC voltage for the contrast requirements of LCD screens. A typical application circuit is shown in Figure 1. Please refer to Application Note 32 for detailed application information beyond what is presented here. The MOSFETs (Q4 and Q5) are alternately turned on with a constant 50% duty cycle signal (L GATE1, L GATE2) at one-half the frequency of the oscillator. In this way each transistor pulses, or excites, the resonant tank on each half cycle. The combination of these two signals appear across the primary winding of the output transformer as a sinusoidal waveform. This voltage is multiplied by the step-up turns ratio of the output transformer and impressed across the lamp. The output transitions are controlled by feedback through the L RTD pin by sensing the voltage at the center tap of the output transformer. Each time this signal reaches the minimum resonant threshold detection point an internal clock pulse is generated to keep the system synchronized. Figure 3 shows some of these representative waveforms at the important nodes of the circuit. The PWM regulator is comprised of a MOSFET (U2-A), inductor L1, and the gate control and drive circuitry as shown in Figure 1. A signal with a constant pulse width of 150ns is applied to the primary of the 2:1 pulse transformer T2, rectified by diode D2, and used to charge the gate capacitance of U2-A, thereby turning it on. The turn off is controlled by discharging this capacitance through MOSFET U2-B. The pulse width of the signal on the gate of U2-B (B OFF) varies according to the amplitude of the feedback signal on LEA-, which is proportional to the AC current flowing in the lamp. This feedback signal is developed by monitoring the current through resistor R6 in the common source connection of the inverter MOSFETs (Q4 and Q5). The lamp current, and therefore brightness, is adjusted by varying the voltage applied to R7 at the brightness adjust control point. Increasing this voltage decreases the brightness.
Note: Please read the Power Sequencing section below prior to using the ML4876. LAMP DRIVER
The lamp driver, sometimes referred to as a lamp inverter, is comprised of a PWM regulator and a Royer type inverter circuit to drive the lamp. The PWM regulator, in a buck configuration, controls the magnitude of the lamp current to provide the dimming capability. Figure 2 shows a simplified circuit to more easily illustrate the operation of the circuit. Due to the presence of the buck inductor, L1, the circuit shown in Figure 2 is essentially a current fed parallel loaded resonant circuit. Lm is the primary inductance of the output transformer, T1, which tunes with the resonant capacitor CR to set the resonant frequency of the inverter. The oscillator frequency is always set lower than the natural resonant frequency to ensure synchronization. The current source IC models the current through the buck inductor L1.
CT
IC
(c)
T1 Lm Lm
COUT T1 1:N
CLOCK
L GATE1
DRAIN-Q5
CR
LAMP
L GATE2
Q4
Q5
DRAIN-Q4
T1-CNTR-PRI SOURCE OF U2-A
Figure 2. Simplified Lamp Driver Circuit
Figure 3. Operating Waveforms of the Lamp Driver Section
7
ML4876
FUNCTIONAL DESCRIPTION
CONTRAST CONTROL GENERATOR
The contrast voltage generator is a separate regulator in a flyback configuration. In conjunction with the external transformer (T3), MOSFET (Q1), diode (D1), and assorted capacitors and resistors, it provides an adjustable DC output contrast voltage necessary to drive LCD screens. The voltage is adjusted by controlling the voltage applied to R5 at the contrast adjustment point. The contrast voltage can be made either positive or negative simply by changing the connection of the external components. The schematic shown in Figure 1 is connected for a negative voltage. Please refer to Application Note 32 for the circuit connection for a positive output voltage. (Continued)
LAMP OUT DETECT
In those cases when there is no lamp connected, or the connection is faulty, the output voltage of the lamp driver circuit will tend to rise to a high level in an attempt to start the nonexistent lamp. The lamp out detect circuit on the ML4876 will detect this condition by sensing the center tap voltage on the primary of the output transformer (T1) on the L RTD pin. When this voltage exceeds 16V, an internal latch is set and the lamp driver goes into a shutdown mode. The logic control pin L ON must be cycled low, then high to reset the latch and return the lamp driver to the normal state.
LOGIC CONTROL
The ML4876 is controlled by a two logic inputs, L ON and ON/OFF. A logic level high on the L ON pin enables just the lamp driver. A logic zero on the L ON pin disable the lamp driver only. A logic level high on the ON/OFF pin enable the complete circuit. A logic level low on the ON/OFF pin puts the circuit into a very low power state.
OSCILLATOR
The frequency of the oscillator in the ML4876 is set by selecting the value of CT. Figure 4 shows the oscillator frequency versus the value of CT. This nomograph may be used to select the appropriate value of CT to achieve the desired oscillator frequency.
POWER SEQUENCING
It is important to observe correct power and logic input sequencing when powering up the ML4876. The following procedure must be observed to avoid damaging the device. 1. Apply the battery power to HVDD 2. Apply the VDD voltage (if HVDD is not used). With HVDD connected this voltage is supplied by the internal regulator on the ML4876. 3. Apply a logic high to the ON/OFF input. This will enable the internal linear regulator to ensure the VDD supply is on (when HVDD is used). 4. Apply a logic high to the L ON input.
LINEAR REGULATOR
A linear voltage regulator is provided to power the low voltage and low current control circuitry on the ML4876. This is typically used when there is no separate 5V supply available at the inverter board. For operation up to 18V the linear regulator is used by connecting HVDD to the input battery voltage. For operation over 18V, a MOSFET and a resistor (Q2 and R1, Figure 1) are connected as shown. The MOSFET is required to stand off the high voltage.
200KHz
100KHz
FREQUENCY
70KHz 50KHz 40KHz 30KHz
20pF
30pF
40pF 50pF CT
70pF
100pF
200pF
Figure 4. Frequency vs. CT
8
ML4876
PHYSICAL DIMENSIONS
inches (millimeters)
Package: R20 20-Pin SSOP
0.279 - 0.289 (7.08 - 7.34) 20
0.205 - 0.213 (5.20 - 5.40) PIN 1 ID
0.301 - 0.313 (7.65 - 7.95)
1 0.026 BSC (0.65 BSC) 0.068 - 0.078 (1.73 - 1.98) 0 - 8
0.066 - 0.070 (1.68 - 1.78)
0.009 - 0.015 (0.23 - 0.38)
SEATING PLANE
0.002 - 0.008 (0.05 - 0.20)
0.022 - 0.038 (0.55 - 0.95)
0.004 - 0.008 (0.10 - 0.20)
ORDERING INFORMATION
PART NUMBER
ML4876CR (END OF LIFE) ML4876ER (OBSOLETE)
TEMPERATURE RANGE
0C to 70C -20C to 70C
PACKAGE
20-Pin Molded SSOP (R20) 20-Pin Molded SSOP (R20)
(c) Micro Linear 1998.
is a registered trademark of Micro Linear Corporation. All other trademarks are the property of their respective owners.
Products described herein may be covered by one or more of the following U.S. patents: 4,897,611; 4,964,026; 5,027,116; 5,281,862; 5,283,483; 5,418,502; 5,508,570; 5,510,727; 5,523,940; 5,546,017; 5,559,470; 5,565,761; 5,592,128; 5,594,376; 5,652,479; 5,661,427; 5,663,874; 5,672,959; 5,689,167; 5,714,897; 5,717,798; 5,742,151; 5,747,977; 5,754,012; 5,757,174; 5,767,653;. Japan: 2,598,946; 2,619,299; 2,704,176. Other patents are pending. Micro Linear reserves the right to make changes to any product herein to improve reliability, function or design. Micro Linear does not assume any liability arising out of the application or use of any product described herein, neither does it convey any license under its patent right nor the rights of others. The circuits contained in this data sheet are offered as possible applications only. Micro Linear makes no warranties or representations as to whether the illustrated circuits infringe any intellectual property rights of others, and will accept no responsibility or liability for use of any application herein. The customer is urged to consult with appropriate legal counsel before deciding on a particular application.
2092 Concourse Drive San Jose, CA 95131 Tel: (408) 433-5200 Fax: (408) 432-0295 www.microlinear.com
DS4876-01
9

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